This invention relates to a throttle valve made of resin used in a throttle body made of resin. Research and development activities have reduced the weight of automobiles to reduce fuel consumption. A conventional throttle body, one of the components of an intake system, is manufactured by aluminum die casting. Efforts have been made in recent years to provide lightweight, low-cost throttle bodies by forming throttle bodies of resins.
This invention relates to a throttle valve made of resin used in a throttle body made of resin. Research and development activities have reduced the weight of automobiles to reduce fuel consumption. A conventional throttle body, one of the components of an intake system, is manufactured by aluminum die casting. Efforts have been made in recent years to provide lightweight, low-cost throttle bodies by forming throttle bodies of resins.
The bore of a throttle body must be formed so that the gap between the bore wall defining the bore of the throttle body and a throttle valve placed in the bore of the throttle body is in the range of 80 to 100 μm. The bore of a conventional throttle body formed by die casting is finished by machining to form the bore to the desired accuracy. If a resin throttle body can be formed such that its bore is formed at an accuracy that insures the gap in the aforesaid range, machining is unnecessary. The roundness of the bore after molding shrinkage, the roundness of the throttle valve (hereinafter, roundness is used to represent variations in diameter), and errors in the inside diameter of the bore must be equal to those of an aluminum throttle valve formed by die casting. It is necessary to prevent the interference between the bore wall and the throttle valve, and an excessive increase in the gap between the throttle body and the throttle valve due to thermal deformation caused by the variation of temperature between a very low temperature and a high temperature exceeding 100° C.
A method of preventing irregular deformation proposed in JP-A No. 169473/1998 places a filler in an orientation in a bore part of a throttle body defining a bore. It is thought that the gap can be reduced when a metal throttle valve formed by machining is placed in such a bore part.
For cost reduction, the throttle valve must be formed of resin to omit a machining process. When both a throttle body having a bore and a throttle valve to be placed in the bore are formed of resins, the throttle body and the throttle valve can be formed of different resins having similar coefficients of expansion. Hence the initial gap between the bore wall and the throttle valve can be substantially maintained. When the throttle valve is formed of a resin having a thermal conductivity lower than those of metals, it is possible to prevent freezing that occurs in metal throttle valves during operation. Even if both the throttle body and the throttle valve are formed of the same resin containing the same amount of filler, the throttle body and the throttle valve will have different coefficients of linear expansion, and deform by different amounts due to the difference between the throttle body and the throttle valve in the orientation of the filler. Consequently, there is the possibility that the throttle valve will interfere with the bore wall, and thus the gap between the bore wall and the throttle valve increases.
Recent internal combustion engine design has tended to reduce the idle throttle valve opening to reduce idle speed. When the idle throttle valve opening is reduced, the possibility increases that contaminants, such as carbon contained in the recirculated exhaust gas, and oils contained in the blowby gas, will adhere to the periphery of the throttle valve. If those contaminants deposited on the throttle valve are solidified by the heat of the internal combustion engine, the throttle valve locks to the bore wall and, in the worst case, the throttle valve will not move even if the accelerator pedal is operated.